End-of-Driving Drivability Analysis
The WVDOH requires all driven piles to bear into or on bedrock. Due to the relatively shallow and hard bedrock usually encountered in West Virginia, the driving stress typically controls pile size. Driving to refusal on bedrock is hard driving and requires hardened steel pile points.
We typically require relatively short piles, and normally perform an end-of-driving analysis. This analysis should be performed using wave equation software such as GRLWEAP. The purpose of the software analysis is to match the driving system and cushion that are to be used by the Contractor to the selected pile size and verify that it does not create driving stress of 45 ksi or greater. Since our piles are relatively short and side resistance is low compared to tip resistance, constant shaft resistance is typically modeled. When pile depths exceed 40 feet, a full drivability analysis using proportional side resistance may be desirable to size a hammer that can set the piles in a reasonable driving time.
Based on the concern of possible damage to the hammer for rock refusal at over 10 blows per inch (BPI), it is desirable to limit driving to about 10 BPI when medium hard and harder rock is encountered. When soft rock is encountered, refusal of 20 BPI is still required. Where relaxation of soft rock is a concern (e.g. claystone, mudstone, and soft shale), the Geotechnical Engineer may judge it necessary to restrike the piles after a minimum of 3 days. For simple span bridges with these softer rock types, two sets of 20 BPI is acceptable. For initial strike and restrike, a single set of 20 BPI, or two sets of 20 BPI can be required by the Geotechnical Engineer based on his analysis. Alternatively, a borehole can be advanced to harder rock when practical in lieu of restriking the piles.
The geotechnical resistance factor of 0.50 (LRFD) is to be used for wave equation analysis without field testing. Consequently, we require a target (ultimate) capacity of 2 times the maximum factored axial pile load to be used in the analysis. The maximum factored load cannot exceed the factored structural resistance of the selected pile. The LRFD resistance factor for hard driving is 0.50 and is used in determining the structural capacity of the pile. However, it is desirable to size piles so that they are a maximum of 90% of the factored structured capacity since the structural resistance is halved while the geotechnical resistance is doubled, making the demand during driving 100% of the strength; and 45 ksi is 90% of 50 ksi which is the maximum stress allowed for piles during driving.
For integral abutments, the boreholes usually are to be predrilled a minimum of 15 feet below the bottom of the pile cap. The boreholes are to be 12 inches in diameter when pre-drilled through soil, and 24 inches when drilled through rock or boulders. All predrilled holes are to be backfilled with clean, dry sand prior to pile placement and driving. For semi-integral abutments, a minimum pile length of 10 feet may be used to provide adequate resistance to pile bounce at refusal. These relatively small diameter bore holes filled with sand allow us to normally use the Meyerhof formula (qs =N160/50) to estimate the percentage of shaft resistance used in the analysis (shaft resistance for piling is not used for axial load demand per our Bridge Design Manual).
When using GRLWEAP software, the end-of-driving analysis (as opposed to a drivability analysis) is selected in the dropdown for constant shaft resistance as shown on Figure 1 below (“Bearing Graph - const. shaft resistance”). As shown on Figure 1, the target capacity should be in the first capacity box. This first value is used by this software to set the constant shaft resistance for all the other target values.
An appropriate toe quake value ranging from 0.04 for hard rock to 0.10 for very soft rock is to be used (see Figure 1).
We prefer screen shots of the input data and output data (Figures 1 and 2) be provided in lieu of voluminous tabulated output data.
The end-of-driving, drivability analysis should be performed by a licensed Geotechnical Engineer practicing in the State of West Virginia.
Typically, the WVDOH requires this analysis to be repeated for 3 to 5 common open end diesel hammers. A typical pile note showing tabulated hammers, fuel settings, and BPI is presented below.
ALL STEEL PILES SHALL BE HP 14X102 STEEL BEARING PILES, PREDRILLED AND DRIVEN, AND SHALL MEET AASHTO M270, GRADE 50 REQUIREMENTS. HARDENED STEEL PILE POINTS SHALL BE USED. THE TARGET CAPACITY IS 720 KIPS PER PILE AT REFUSAL IN BEDROCK (THE MAXIMUM FACTORED LOAD IS 360 KIPS PER PILE). THE CONTRACTOR SHALL PREDRILL ALL THE PILE BOREHOLES A MINIMUM OF 15 FEET BELOW THE BOTTOM OF THE PILE CAP AND BACKFILL WITH SAND. BASED ON THE BORINGS DRILLED AT THE SITE, SOME OF THE BOREHOLES WILL EXTEND AS MUCH AS 4.5 FEET INTO HARD, ABRASIVE SILTSTONE TO REACH THE REQUIRED MINIMUM 15 FEET. THE CONTRACTOR SHALL HAVE EQUIPMENT CAPABLE OF DRILLING THE REQUIRED DEPTH INTO HARD, ABRASIVE SILTSTONE.
REFUSAL IS DEFINED IN BLOWS PER 1 INCH (BPI) OF PENETRATION USING A POWER HAMMER AS FOLLOWS:
||BPI @ TARGET
|DELMAG D 19-42
||Max-2 (2nd Highest)
HAMMERS LARGER THAN 47,126 FT-LBS SHALL NOT BE ALLOWED WITHOUT PRIOR APPROVAL OF THE ENGINEER. IF THE CONTRACTOR ELECTS TO USE A HAMMER THAT IS NOT INCLUDED IN THE ABOVE TABLE OR IF A FUEL SETTING OTHER THAN THOSE LISTED ABOVE IS USED, THE CONTRACTOR SHALL SUBMIT DRIVING CRITERIA USING A WAVE EQUATION ANALYSIS, SUCH AS GRLWEAP. THE DRIVING CRITERIA SHALL BE SUBMITTED TO THE ENGINEER AT LEAST 14 DAYS PRIOR TO THE INSTALLATION OF THE FIRST PILE AND SHALL INCLUDE THE FUEL SETTING(S), AND THE REQUIRED BLOWS PER 1 INCH OF PENETRATION TO ACHIEVE THE TARGET CAPACITY, AND THE MAXIMUM DRIVING STRESS AT REFUSAL. A MINIMUM OF 6 BPI MAY BE ALLOWED AS PRACTICAL REFUSAL BY THE ENGINEER BASED ON THE WAVE EQUATION ANALYSIS. THE ANALYSIS SHALL DEMONSTRATE THAT THE TARGET CAPACITY CAN BE ACHIEVED WITHOUT DAMAGING THE PILES OR EXCEEDING 45 KSI MAXIMUM COMPRESSIVE STRESS.
ALL PREDRILLED HOLES SHALL HAVE A MINIMUM DIAMETER OF 24 INCHES AND SHALL BE BACKFILLED WITH CLEAN, DRY SAND PRIOR TO DRIVING.
ANY WAVE EQUATION ANALYSIS PERFORMED BY THE CONTRACTOR AND ASSOCIATED COSTS DUE TO DELAYS, DRIVING EQUIPMENT, OR MOBILIZATION SHALL BE AT THE CONTRACTORS EXPENSE. THE COST OF THE PILE POINTS AND SAND BACKFILL SHALL BE INCLUDED IN ITEM 616005-XXX, HP 14X102 STEEL BEARING PILE, PRE-DRILLED AND DRIVEN.